Cystatin C: what it is, what it tells you and when it’s tested

Cystatin C is a small protein that every cell in your body makes at a steady pace. Your kidneys filter it out of the blood. If they filter less, cystatin C rises. Unlike creatinine, it hardly depends on how much muscle you have.

When it’s tested

Doctors order it when the eGFR calculated from creatinine may be wrong. That happens in people with a lot of muscle or who take creatine, in people with very little, for example with age or after a long illness, and with a diet very high or very low in protein.

They also order it when they need to know exactly how well the kidneys filter: to confirm kidney disease, or to choose the dose of some medicines. The lab combines it with creatinine into a more precise eGFR.

For most people who feel well, eGFR from creatinine and urine albumin are enough to check the kidneys. Cystatin C is a second step, when there is doubt. That is why Rosa does not test it. Instead it measures creatinine, eGFR and urine albumin.

What it tells you, and what it does not

If it is high, your kidneys usually filter less. On its own, or together with creatinine, it gives an estimate of kidney filtration, the eGFR, that is not affected by muscle.

When the eGFR from creatinine and the one from cystatin C are very different, the difference itself is useful information for your doctor.

It does not say why the kidneys filter less, and it does not pick up albumin leaking into the urine. And some things raise it even with healthy kidneys: smoking, inflammation, steroids, the thyroid.

That is why it is read together with other tests:

Normal cystatin C levels

The lab decides what is normal, and it depends on the method. In adults the upper limit is usually close to 1 mg/L. For older people many labs print a higher limit.

The report often also gives the eGFR calculated from cystatin C, or from cystatin C and creatinine together. That is what matters most to your doctor: below 60 the kidneys filter less than normal.

You usually do not need to fast. Follow your lab’s instructions all the same.

If your cystatin C is high or low

What raises cystatin C

The most common cause comes first. The others explain a high value with healthy kidneys.

What can skew the result

Compared with creatinine, many things matter little. A few do.

When to worry

Talk to your doctor if:

Frequently asked questions

What are normal cystatin C levels? It depends on the lab and the method. In adults the upper limit is usually close to 1 mg/L, higher in older people. The range on your own report is what counts.

What does high cystatin C mean? Usually that your kidneys filter less. Sometimes steroids, the thyroid, smoking or inflammation are behind it. Your doctor looks at the eGFR, creatinine and urine albumin.

Is cystatin C or creatinine better? They complement each other. Creatinine depends on muscle, cystatin C on other things. Together they give the most precise estimate of how well your kidneys filter.

Does cystatin C say anything about the heart? People with a higher cystatin C have more heart events on average, especially in older age. But it is not a heart test: it mainly shows how your kidneys are doing.

Does Rosa test cystatin C? No. Rosa measures creatinine, eGFR and urine albumin, which are enough to check the kidneys in most people. If your doctor wants cystatin C, they order it separately.

Sources